Abstract

The Quaternary-aged (1.59 – 0.51 Ma) intraplate, monogenetic, basaltic South Auckland Volcanic Field consists of at least 82 volcanic centres that span an area of 300 km². This study focusses on the volcanic and sedimentary histories of the Onewhero and Kellyville maars, interpreted from additional geological observations, which build upon previous geological studies, and gravimetric and magnetic surveys.

New geological observations of the Onewhero maar reveal the presence of diatomaceous sediment, and at least one lava flow, that probably originated from the nearby Klondyke cone to the south, and which occupies a significant area of the crater floor. A geological investigation of the Kellyville maar reveals a crater that has been partially filled by a thick accumulation of basalt and scoria and overlain by Karapiro Formation sediments and diatomite. Available borehole stratigraphy suggests that the Glass Hill basalt cone present in the Kellyville maar was an early-stage volcanic product rather than a late-stage product as previously thought.

Gravimetric surveys in the Onewhero and Kellyville maars revealed contrasting crater fill deposits. Raw gravity data was acquired in the field, corrected for drift and then reduced to a Bouguer anomaly. The Onewhero maar is characterised by an anomaly of -5.8 mGal that has a concave profile. The post-eruption geological body present at Onewhero is two-dimensionally modelled with a density of 1.3 g/cm³, and has a maximum thickness of 100 m in the northwest of the crater. The Kellyville maar is characterised by an anomaly of +2.5 mGal that has a convex profile. The dominant geological body present in the crater is a lava-lake deposit that is two-dimensionally modelled with a density of 2.9 g/cm³, and has a maximum thickness of 60 m.

The total magnetic field strength of the Onewhero and Kellyville maars was recorded with a proton magnetometer and the values were mapped in ArcMap. In the Onewhero maar, two subdued anomalies were outliers. A positive magnetic anomaly (213 nT) with a broad crescent shape was identified in the middle of the crater and interpreted to be either an extrusive feeder dyke or an accumulation of highly magnetic volcanic sediment deposited in a topographic trough. A circular negative anomaly (-1057 nT) in the northwest of the crater is an accumulation of lake sediment with a low magnetic susceptibility. In the Kellyville maar, two contrasting anomalies were observed. The strongly positive anomaly (1275 nT) corresponds to the geographical position of Glass Hill, a basaltic cone. A strong negative anomaly (-3315 nT) in the middle of the crater corresponds to a known deposit of diatomite.

Overall, the structure of the Onewhero maar, as inferred by this investigation, is similar to many other maars globally and its sedimentary fill is remarkably similar to the Dottingen maar in Germany. The Kellyville maar has a more unique structure but can be compared to the Domain maar in the nearby Auckland Volcanic Field.